How should we understand that a concentration difference is necessary for a distillation process to take place?
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In a book on distillation, there is a passage stating that when the vapor and liquid phases come into contact, there must be a concentration difference and a temperature difference; in other words, the concentration of light components in the rising vapor must be lower than that in the vapor at equilibrium on that tray. How to understand the principle of concentration difference? Why is the concentration of light components in the rising gas phase lower than that at equilibrium?
In a distillation column, the concentration of light components increases gradually from bottom to top; therefore, the amount of light components in the rising vapor is definitely less than that in the vapor from the previous tray. Concentrate gradually.
You can think of the vapor-liquid exchange between the trays as a process of mutual mass and heat transfer; that book explains it very clearly